The increase in species richness or biodiversity that occurs from the poles to the tropics, often referred to as the latitudinal gradient in species diversity (LDG), is one of the most widely recognized patterns in ecology.
Species diversity varies systematically across the globe with latitude, longitude, and altitude (or its equivalent, depth, in the oceans). The trend toward higher species diversity in the tropics is perhaps the most conspicuous biogeographic pattern in nature. In most marine groups, diversity is maximal in the Indo-West Pacific.
Many mountain ecosystems are host to higher species richness and levels of endemism than adjacent lowlands. Mountains at lower altitudes can support exceptional biodiversity, due to compression of a wide range of ecosystems into a relatively short distance. Mountains also often provide islands of suitable habitat, isolated from unfavourable surrounding lowlands.